Visceral larva migrans, or VLM, is a condition in humans usually attributed to the larval migration of the common dog roundworm, Toxocara canis. The spectrum of illness resulting from this condition ranges from sub-clinical or only non-specific mild symptoms to asthma, convulsions, blindness and death. The occurrence of this condition depends upon the availability of an infected canine source, a suitable environment for the embryonation and survival of the roundworm eggs, and ingestion of the eggs by humans. That VLM can and does occur in urban, temperate areas as evidenced by the four percent prevalence rate of infection found by these investigators in a pilot study of over 1,000 children conducted in 1978. The recent development of a reliable serological test, the enzyme-linked immuno-sorbent assay (ELISA) makes it now possible to determine if VLM may be a signaficant public health problem for New York City. The proposed study, which has both retrospective and prospective elements, will utilize ELISA as its screening tool to establish the prevalence of infection and disease with VLM in children. Approximately 10,000 venous blood specimens collected during the Lead Poisoning Control Program will be screened for VLM by ELISA. It is expected that of these high-risk New York City youngsters, about 400 will be positive (cases). All these cases and an equal-sized group of controls will then be interviewed at home, examined and blood-tested at District Health Stations. From comparison of the data obtained, we expect to clarify the host and environmental factors which influence the contraction and manifestations of visceral larva migrans. The pathogenicity of this condition relative to infection will be understood for the first time. Furthermore, the protective or predictive significance of antibody possession for Toxocara canis will be understood.